"We want to turn people's cells into drug factories, giving them the genetic information they need to produce their own treatment," says Matthew Scholz of Immusoft in Seattle. Immusoft is focusing on B-cells, a component of the immune system. One of their roles is to produce antibodies against infections or unwanted foreign substances such as toxins. In 2009, David Baltimore and colleagues at the California Institute of Technology in Pasadena showed that stem cells from bone marrow could be engineered to develop into B-cells that made antibodies against HIV.

Imagine never having to take a pill again. Instead, mini drug factories hidden inside your bones, and made from your own immune cells, would churn out personalised drugs and other molecules designed to keep you fit and healthy. Such a factory has been created in mice, and could soon be tested in humans to treat HIV.

The Immusoft researchers have built on that work, extracting immature B-cells from human blood and treating them with a virus that inserts the genetics code for a new protein. When the modified cells were injected into the bloodstream of mice, some migrated to the bone marrow. Her they began making the proteins they had been engineered to make a class of so-called broadly neutralising antibodies against HIV. "I think this is potentially very important," says Carl June at the University of Pennsylvania, Philadelphia, who has been engineering another part of the immune system, T-cells, to fight HIV.

"But whether this has clinical use depends on the efficiency of gene transfer, expression and secretion of the antibody, and most importantly the level of secreted protein in body fluids."